Modern printed circuit board technology typically employs a multi-layered approach to efficiently distribute large groups of signal paths. For example, and referring to FIG. 1, an electronic assembly 10 often includes a flip-chip bonded semiconductor device 12 mounted within a ball grid array (BGA) package 14 that in-turn, interfaces with a PCB assembly 16. While different in size and scale, both the package and the PCB assembly employ multi-layer circuit board technology.
Multi-layer circuit boards, whether large PCBs or small micro-boards, typically utilize a plurality of stacked layers 20 for efficient signal routing. The layers 20, as shown in FIG. 2A, each generally include a flat dielectric substrate 22 and a thin conductor 24. The conductor provides a ground/power plane, and is often deposited on the substrate. The layers are laminated in a stacked arrangement with relative precision to preserve strict planarity specifications. Vias 26 formed transverse to the substrate and conductor portion enable layer-to-layer signal routing. The vias are electrically isolated from the conductive plane by a dielectric-filled and metallization-free area known generally as an anti-pad 28.
Typically, the entire structure is fabricated according to design rules appropriate for the specific process. Deviating from the standard design rules for a given process often results in additional cost, and/or unexpected problems. One design rule governing the formation of anti-pads constrains the diameter of the anti-pad in an effort to minimize sagging, or non-planarity in the area of the anti-pad (illustrated in phantom in FIG. 2B). We have found that enlarging the anti-pads enhances performance for high speed signals. Unfortunately, to do this conventionally by customizing the design outside of the conventional design rules often adds undue cost.
What is needed and as yet unavailable is a circuit board structure that allows for the expansion of anti-pads without violating standard fabrication design rules. The circuit board structure described herein satisfies this need.